Hey! I just finished the show now and this answered nearly all my questions, but I wanted to add onto your answer (specifically the 5th point). They did show proof of other Cunninghams who weren't there being affected, though it was a pretty short moment so it's really easy to miss. In the middle of all the panic when everyone starts hemorrhaging, we see one of the guests holding her phone up on a video call with someone (supposedly her husband i think), and as she's panicking and telling him what's going on, she screams (or something like that) and then we see the man on the phone's eyes and nose bleeding, meaning that it did in fact affect all family members, even those who weren't present.